Thaw the puff pastry at room temperature according to the package instructions. Preheat the oven to 400 degrees.
Cut the white parts off the bottom of the leeks. Slice into pieces and place in a strainer. Make sure to rinse them well in cool water. Leeks tend to be very dirty.
Heat the butter in a large skillet over medium heat. Add the leeks and cook for about 3-5 minutes, until leeks have softened and are starting to brown. Add the mushrooms. You may have to do this in batches if they don't all fit in the pan. Add what will fit, then wait for them to cook down and add the rest.
Stir mushrooms to mix in the leeks. Cook for 5-10 minutes, until mushrooms have released their liquid and the liquid has evaporated. Add the sherry and stir until it has evaporated.
Add the fish sauce, salt and pepper and stir to combine. Remove from heat and set aside.
Lay the puff pastry sheets on two parchment paper lined baking sheets. Place half the mascarpone sauce mixture on each and spread, leaving 1" on the sides.
Cover the cheese mixture with the sauteed mushrooms, leaving 1" on the side. Fold up the edges and pinch the corners together (see photo below).
Bake for 20-25 minutes, until puff pastry is golden brown.Let cool for 5-10 minutes, then slice into pieces.